1. Field of the Invention
The present invention generally relates to semiconductor memory devices, and particularly relates to a nonvolatile semiconductor memory device that uses nonvolatile memory cells.
2. Description of the Related Art
There are cases in which it is not desirable to let users use the data stored in memory indefinitely for reasons relating to copyright or the like. Such cases may include the case in which music or image information is temporarily downloaded by use of a portable terminal, the case in which free sample data is used for a limited time period, the case in which software with an expiration date for use is used, etc.
In order to erase all the stored data after a predetermined time period in a flash memory that is a nonvolatile semiconductor memory device, a timer circuit is generally provided inside the device for the purpose of timekeeping. When the data retention period is a long period such as one month, however, a circuit of enormous size is required to implement the timer that can time such a long time period. Such configuration is difficult to implement as a semiconductor device.
Accordingly, the present invention is aimed at providing a nonvolatile semiconductor memory device that can erase stored data after a predetermined time period without the use of a timer circuit.
Non-patent Documents 1 and 2 disclose the background technology of the present invention.
Non-Patent Document 1
Horiguchi and five others, “A Direct Tunneling Memory (DTM) Utilizing Novel Floating Gate Structure,” Technical Digest of International Electron Devices Meeting 1999, U.S., IEEE, p 922
Non-Patent Document 2
Usuki and two others, “Advantage of a quasi-nonvolatile memory with ultra thin oxide,” Abstract of International Conference on Solid State Devices and Materials 2001, The Japan Society of Applied Physics, p 532